Titanium dioxide (TiO2) is an important pigment in the manufacture of paints, plastics, and coatings. There has been a considerable research effort to make titanium dioxide pigments with desirable properties (i.e., fine particle size, gloss and durability).
Titanium dioxide is produced commercially in two crystalline polymorphic forms, namely the rutile form which can be produced by both the chloride and sulfate processes and the anatase form which is produced by the sulfate process. These processes are well known in the art.
Anatase and rutile forms of TiO2 differ not only in their crystallographic structures but also in the type and amount of surface and other impurities resulting from their differing methods of production. These impurities can cause particle agglomeration, effect particle size distribution and ultimately pigment gloss and/or durability.
The prior art discloses adding different chemicals before or during titanium dioxide formation to prevent unwanted growth and agglomeration of titanium dioxide that may effect gloss and durability in the end product. For example, in the chloride process, titanium dioxide is made by reacting titanium tetrachloride (TiCl4) with oxygen. The prior art describes adding aluminum trichloride (AlCl3) with TiCl4 to promote substantially rutile titanium dioxide formation. One way to reduce particle size and agglomerates is to add a silicon halide (i.e. silicon tetrachloride). The reaction between silicon tetrachloride (SiCl4) and oxygen results in the formation of silica that reduces the sintering rate of titania and results in smaller particles and fewer agglomerates. Unfortunately, SiCl4 addition promotes unwanted anatase that may result in a less durable pigment.
Other prior art references disclose treating titanium dioxide after the pigment is produced. For example, the prior discloses improving gloss and durability of titanium dioxide pigment by coating or wet treating the pigment with inorganic compounds. Wet treatment processes are well known in the art and typically involve precipitating hydrous oxide coatings onto the titanium dioxide pigment from metal salt solutions. Some pigment coatings include oxides and hydroxides of alumina, silica, zirconium and phosphate. These wet treatment processes can result in a more durable pigment with improved gloss.
In general, inorganic coatings are known to improve durability, dispersibility and/or gloss of the titanium dioxide pigment. However, some coatings cause the formation of agglomerates that tend to cause difficulties in dispersing pigments into various compositions thereby reducing gloss and durability.
Based on the foregoing, there is still a need for titanium dioxide pigments that have improved gloss and/or durability. These pigments are useful in the manufacture of paints, plastics, and other coating systems.
The present invention provides titanium dioxide pigments with reduced particle size, improved gloss and/or durability and methods of making these pigments. Preferred titanium dioxide pigments of the present invention are substantially free from sulfate.
In one embodiment, the present invention provides a method for preparing titanium dioxide pigment having improved gloss and/or durability comprising: treating titanium dioxide with a plurality of metal salts under conditions suitable to form the titanium dioxide pigment having improved gloss and/or durability, wherein at least one of the plurality of metal salts is capable of providing a monovalent anion.
In another embodiment, the present invention provides a method for preparing titanium dioxide pigment, having improved gloss and/or durability comprising: wet treating titanium dioxide with an alumina compound, a zirconia compound capable of providing a monovalent anion and a phosphate compound under conditions suitable to form the titanium dioxide pigment having improved gloss and/or durability.
In still another embodiment, the present invention provides a method for preparing titanium dioxide pigment having improved gloss and/or durability, comprising: wet treating an aqueous slurry of titanium dioxide with an alumina compound, a zirconia compound capable of providing a monovalent anion and a phosphate compound under conditions suitable to form the titanium dioxide pigment having improved gloss and/or durability.
In one exemplary embodiment, the present invention provides a titanium dioxide pigment having improved gloss and/or durability produced by a method comprising: wet treating titanium dioxide with an alumina compound, a zirconia compound capable of providing a monovalent anion and a phosphate compound under conditions suitable to form the titanium dioxide pigment having improved gloss and/or durability.
In another exemplary embodiment, the present invention provides a titanium dioxide pigment having improved gloss and/or durability produced by a method comprising: wet treating an aqueous slurry of titanium dioxide with an alumina compound, a zirconia compound capable of providing a monovalent anion and a phosphate compound under conditions suitable to form the titanium dioxide pigment having improved gloss and/or durability.
In yet another exemplary embodiment, the present invention provides, a substantially sulfate-free titanium dioxide pigment having improved gloss and/or durability comprising: titanium dioxide base surface treated with an alumina compound, a zirconia compound and a phosphate compound.
For a better understanding of the present invention together with other and further embodiments, reference is made to the following description taken in conjunction with the examples, the scope of which is set forth in the appended claims.
In preparing the preferred embodiments of the present invention, various alternatives may be used to facilitate the objectives of the invention. These embodiments are presented to aid in an understanding of the invention and are not intended to, and should not be construed to, limit the invention in any way. All alternatives, modifications and equivalents that may become obvious to those of ordinary skill upon reading the present disclosure are included within the spirit and scope of the present invention.
This disclosure is not a primer on TiO2 pigment production, basic concepts known to those skilled in the field of TiO2 production have not been set forth in detail. Concepts such as choosing appropriate reactor, additives for TiO2 pigment production, or suitable conditions for wet treatment (i.e., temperature, pH, residence times, etc) are readily determinable by those skilled in the industry and are generally described in the prior art. Attention is therefore directed to the appropriate texts and references known to those skilled in the art with regard to these matters.
The titanium dioxide pigments of the present invention are prepared starting with titanium dioxide base particles. Titanium dioxide base particles are produced commercially in two crystalline forms, namely the rutile form which is usually produced by the chloride and sulfate processes and the anatase form which is usually produced by the sulfate process. Both of these processes are well known to those skilled in the art. The present invention is applicable to both the rutile and anatase forms of TiO2 pigment.
Preferably, the base titanium dioxide pigment is substantially free from metal salts capable of generating polyvalent anions i.e., sulfate anions (SO42xe2x88x92). Thus, the present invention contemplates treating titanium dioxide base with at least one metal salt capable of generating monovalent anions. Suitable metal salts capable of generating monovalent anions include silicon halides, such as SiBr4, SiF4, SiI4, SiCl4, alumina halides, i.e., AlCl3, AlBr3 and AlI3, and zirconia halides, such as, ZrCl4. These metal salts can be introduced into the reactor before or during TiO2 formation by methods known in the art.
As used herein, substantially sulfate-free means that the TiO2 is essentially free from the sulfate anions adsorbed on the base structure of the TiO2. Substantially sulfate-free TiO2 base can be made by limiting sulfate content of reactants, i.e., using metal salts capable of generating monovalent anions. Alternately, substantially sulfate-free TiO2 base can be made by washing the TiO2 base to remove structural sulfate ions. Preferably, the TiO2 pigment is at least 99% free from sulfate and more preferably at least 99.9% free from sulfate or higher.
In one embodiment of the present invention, after the TiO2 base particles are produced, the base is subjected to wet milling or grinding by methods known in the art to provide a base of substantially uniform particle size. Preferably, the base particle has a size range of less than about 1.0 micron, and more preferably, between about 0.1 microns and about 0.5 microns, and most preferably between about 0.1 microns and about 0.3 microns.
Preferably, base TiO2 particles are incorporated into an aqueous slurry, where it is wet treated. Slurries of titanium dioxide base can be made by methods known in the art. Preferably, the slurry has a TiO2 solids content of greater than about 5%, more preferably, less than about 75% and most preferably, the TiO2 slurry has a TiO2 solids content of greater than about 30%.
Typically, the pH of the slurry can be determined by those skilled in the art and adjusted to a desired pH range using methods known in the art. For example, if adjustment of the pH of the slurry becomes necessary, such adjustment can be made by simply adding a suitable acid or suitable base. Suitable acids include water soluble acids such as hydrochloric acid, nitric acid, and the like. Preferably, the pH of the initial slurry is at least about 3.4. Suitable bases include water soluble alkaline bases such as ammonia, sodium hydroxide or other suitable alkaline bases.
In the most preferred embodiment of the present invention, the titanium dioxide base is wet treated. Methods of wet treatment are known in the art and typically involve precipitating hydrous oxide compounds onto the titanium dioxide base. Compounds suitable for use in the present invention include alumina, zirconium and phosphate compounds. These compounds are capable of providing oxide or hydrous oxide coatings on the base titanium dioxide particle.
While not wishing to be bound to any one particular theory, it has been discovered that soluble metal salts with polyvalent anions (i.e., sulfate ions (SO42xe2x88x92)) tend to remain structurally on the pigment due to ion exchange with the surface of the pigment and coating. Conventional washing does not remove these polyvalent anions. Soluble polyvalent anions present after the titanium dioxide pigment is surface treated can cause flocculation, reducing dispersibility and thus opacity of the end product, as well as gloss and/or durability of the pigment. Metal salts with monovalent anions (i.e., chlorine, bromine, acetate, fluorine) have reduced tendency to flocculate, thus dispersibility, opacity of the end product, as well as gloss and/or durability of the titanium dioxide pigment are improved.
Alumina compounds added to the slurry include hydrous alumina compounds such as for example, water soluble alkali metal aluminates. Some water soluble alkali metal aluminates, include but are not limited to sodium aluminate or potassium aluminate. Some other alumina compounds include aluminum halides, i.e., aluminum chloride, aluminum bromide, and the like. Most preferably, the alumina compound is substantially sulfate-free and comprises sodium aluminate.
The weight percent of the alumina compound can vary depending on the wet treatment. Preferably, the alumina compound is added to the slurry in an amount to provide from about 0.5% to about 5.0% Al2O3 by weight, more preferably from about 1.0% to about 3.0% by weight Al2O3 based upon the total weight of the titanium dioxide pigment.
For purposes of the present invention, any water soluble zirconia compound capable of providing monovalent anions can be used under the operating conditions of the methods of the present invention. Zirconia compounds suitable for use in the present invention include zirconia comprising monovalent anions such as for example, zirconia acetate, zirconia halides, i.e., zirconium chloride, zirconium oxychloride, and the like. Most preferably, the zirconia compound is substantially sulfate-free and comprises zirconium oxychloride.
The weight percent of the zirconia compound can vary depending on the layer precipitated on the titanium dioxide base. Preferably, the zirconia compound is precipitated in an amount of from about 0.1% to about 3.0% by weight ZrO2, more preferably from about 0.1% to about 0.4% by weight ZrO2 based on the total weight of the titanium dioxide pigment.
Suitable phosphate compounds added to the slurry include any water soluble phosphate compound capable of providing an insoluble phosphate for deposition upon the titanium dioxide base under suitable operating conditions. Suitable phosphate compounds for use in the present invention include water soluble phosphate compounds, such as for example, tetrapotassium pyrophosphate, sodium polyphosphate, tetrasodium pyrophosphate, sodium tripolyphosphate, potassium tripolyphosphate, sodium hexametaphosphate, phosphoric acid, and the like. Most preferably, the water soluble phosphate compound is substantially sulfate-free and comprises sodium hexametaphosphate.
The weight percent of the phosphate compound can vary depending on the wet treatment. Preferably, the phosphate compound provides from about 0.1% to about 1.0% by weight P2O5, more preferably from about 0.1% to about 0.5% by weight that provides insoluble P2O5 based on the total weight of the titanium dioxide pigment.
The temperature of the slurry can easily be determined by those skilled in the art to facilitate the wet treatment process. The slurry can be heated using conventional heating means known by those skilled in the art, such as for example, by steam.
The order of addition is not critical in the present invention. Thus, in one embodiment, the present invention contemplates additions of alumina, zirconia and phosphate compounds in any order.
Upon completion of the addition of the alumina, zirconia and phosphate compounds. The resultant titanium dioxide pigment is recovered by filtration. Preferably, the pigment is washed substantially free of salts (if any) adhering to the pigment and/or not adhering to the pigment, dried and then subjected to final milling using fluid energy milling techniques known in the art. Preferably, the washed and dried pigment is micronized in a steam or air micronizer at intensities known by those skilled in the art.
Optionally, a polyol such as for example, trimethylolpropane (TMP), trimethylolethane (TME) or pentaerythritol, and the like, can be added to the pigment during steam or air micronization. In the most preferred embodiment, TMP is added in an amount of from about 0.2% to 0.4% based on the weight of the titanium dioxide pigment.
It has unexpectedly been discovered that the coated or wet treated pigments produced by the methods of the present invention have improved gloss and/or durability. These pigments are multipurpose and suitable for use in paints, plastics, and coatings, where both durability and gloss are needed.
Gloss is determined by methods known in the art. Preferably, gloss is determined by incorporating the pigment into paint and measuring the gloss using a gloss meter. In the most preferred method, the pigments made by the methods of the present invention are incorporated into acrylic based paints at 40% PVC (pigment volume concentration) and the gloss measured.
Durability of the pigments of the present invention can be determined by methods known in the art. Some methods of measuring durability include measuring photocatalytic activity of the pigment, acid solubility of the titanium dioxide base, natural exposure and weatherometer testing. Most preferably, durability is measured by natural exposure or weatherometer testing. In the later case, total gloss values are the sum of the individual gloss readings. The higher the total, the better the gloss and overall durability of the pigment.
Weatherometer testing involves incorporating the pigments into, for example, paint and subjecting the paint to exposure in an Atlas Weatherometer and then comparing the coated pigments with other pigments i.e., sulfate containing pigments. The pigments are then rated as to durability.
As used herein, improved gloss includes increases in gloss when compared to pigments produced with polyvalent anions, i.e., sulfate. Increased gloss includes increases from about 1% to about 100% or higher, over pigment with polyvalent anions, i.e., sulfate.
Improved durability includes increases in durability when compared to pigments produced with polyvalent anions, i.e., sulfate. Increased durability includes increases from about 1% to about 100% or higher, over pigment with polyvalent anions, i.e., sulfate.
Having now generally described the invention, the same may be more readily understood through the following reference to the following examples, which are provided by way of illustration and are not intended to limit the present invention unless specified.